1. Field of the Invention
The present invention relates to a position detection sensor and more particularly to a position detection sensor having a linear scale for detecting a traveling amount, traveling speed and absolute position of an object to be measured.
2. Description of Related Art
A photoelectric position detection sensor which employs a linear scale is, as is known, configured such that a relative traveling amount between an optical device and the linear scale and the position of the optical device relative to the linear scale can be detected by reading optically a pattern provided on the linear scale by the optical device which moves relative to the linear scale.
FIGS. 13A to 13C and 14B to 14C are drawings illustrating the configurations of conventional position detection sensors which employ a linear scale, of which FIG. 13A illustrates a reflective type position detection sensor and FIG. 14A illustrates a transmissive type position detection sensor.
In the reflective type position detection sensor, as is shown in FIG. 13A, high reflectivity portions (white or the like high luminance portions) 202 and low reflectivity portions (black or the like low luminance portions) 204 are provided alternately on a linear scale 200. In addition, a reflective photointerrupter 210 is disposed in such a manner as to be opposed to the reflective linear scale 200, and the photointerrupter 210 moves relative to the linear scale 200. For example, the linear scale 200 is attached to a stationary part, while the photointerrupter 210 is provided so as to move in conjunction with a moving object to be measured.
As is shown in FIG. 13B, a light projecting part (a light emitting device 212) for projecting light to the linear scale 200 and a light receiving part (a light receiving device 214) for receiving light projected from the light projecting part and reflected on the linear scale 200 are provided on the same surface side of the reflective photointerrupter 210. A pulse signal made up of a high level voltage and a low level voltage as is shown in FIG. 13C is outputted from the photointerrupter 210 (the light receiving device 214) as a detection signal every time the photointerrupter 210 travels a distance equaling one pitch which is made up of a high reflectivity portion 202 and a low reflectivity portion 204 lying adjacent thereto on the linear scale 200.
In the transmissive type position detection sensor, as is shown in FIG. 14A, slits (light transmissive portions) 222 and light blocking portions 224 are provided alternately on a linear scale 220. In addition, a transmissive photointerrupter 230 is disposed relative to the transmissive linear scale 220 in such a manner as to sandwich the linear scale 220 between its light emitting part 232 and light receiving part 234, and the photointerrupter 230 travels relative to the linear scale 220. For example, the linear scale 220 is attached to a stationary part, while the photointerrupter 230 is provided so as to move in conjunction with a moving object to be measured.
As is shown in FIG. 14B, the light projecting part 232 (a light emitting device 236) for projecting light to the linear scale 220 and the light receiving part 234 (a light receiving device 238) for receiving light which was projected from the light projecting part 232 and which passed through the linear scale 220 are provided on the transmissive photointerrupter 230 in such a manner as to be opposed each other across the linear scale 220. A pulse signal made up of a high level voltage and a low level voltage as is shown in FIG. 14C is outputted from the photointerrupter 230 (the light receiving device 234) as a detection signal every time the photointerrupter 230 travels a distance equaling one pitch which is made up of a slit 222 and a light blocking portion 224 on the linear scale 220.
JP-A-2002-048599 and JP-A-2005-024276 propose a technique in which a liquid crystal panel (a liquid crystal scale) is used as the linear scales described above, so that the patterns of the linear scales are realized by the liquid crystal panel through control of the liquid crystal.
In the reflective type position detection sensor described above, however, since reflected light which was originally projected from the light projecting part of the optical device and was then reflected on the linear scale is detected, the output level of a detection signal is generally low, and S/N ratios differ from product to product. Because of this, no stable detection signal can be obtained, and also it is difficult to set a slice level based on which it is determined whether a detection signal is a high or low level signal.
In addition, in the transmissive type position detection sensor described above, although the output level of a detection signal is high because light that has passed through a slit in the linear scale is detected, the shape of the slits affects detection accuracy. In addition, since the light emitting part and the light receiving part of the optical device (the photointerrupter or the like) are disposed in such a manner as to be opposed to each other across the linear scale, the width of the sensor becomes large.
Further, the linear scales shown in FIGS. 13A and 14A are not intended for detecting absolute positions but intended for detecting relative positions (traveling amounts) and traveling speeds, and hence, they cannot detect absolute positions directly from detection signals. In addition, although there is a case where a linear scale employing a resistance value approach is used as a device for detecting an absolute position, an error is caused depending upon a traveling direction of a brush part for detecting a resistance value, leading to a problem related to accuracy.